Researchers have shown that male mice that eat yogurt lug around bigger testicles, inseminate partners faster, and produce more offspring than those who don't. And yogurt-eating females give birth to larger and healthier litters.

The bigger balls also gave the mice what the researchers describe as "mouse swagger," which is probably the single greatest mental image you'll have all day.

When MIT researchers Susan Erdman and Eric Alm caught wind of a Harvard study that showed yogurt could help prevent age-related weight gain, they decided to try to replicate the findings in mice. The pair fed 40 males and 40 females either a standard mouse diet or a diet high in fat and low in fiber (the latter was meant to simulate a junk-food-laden human diet). They then supplemented the diets of half the mice from each group with yogurt.

Erdman and Alm had originally intended to study the effects of yogurt on obesity rates and other health issues, but they soon noticed something... different about their yogurt-fed mice. Something they hadn't anticipated at all. SciAm's Elie Dolgin explains:

Then the researchers spotted some­thing particular about the males: they projected their testes outward, which endowed them with a certain "mouse swagger," Erdman says. On measuring the males, they found that the testicles of the yogurt consumers were about 5 percent heavier than those of mice fed typical diets alone and around 15 percent heavier than those of junk-eating males.

More important, that masculinity pays off. In mating experiments, yogurt-eating males inseminated their partners faster and produced more offspring than control mice. Conversely, females that ate the yogurt diets gave birth to larger litters and weaned those pups with greater success. Reflecting on their unpublished results, Erdman and Alm think that the probiotic microbes in the yogurt help to make the animals leaner and healthier, which indirectly improves sexual machismo.